Coal is the largest source of human-generated mercury emissions in the United States. Coal-fired power plants release about 48 tons of mercury annually, according to EPA data. In contrast, the total amount of mercury in crude oil processed in the U.S. annually is less than five percent of the amount contained in U.S. coal produced and consumed annually.
Mercury concentrations in crude oil have been reported from as low as <1 ng/g to as high as 50,000 ng/g of oil (see e.g., FIG. 1). Some of the variability observed in crude oil mercury data is due to difficulties encountered in performing the analyses. A wide variety of measurement techniques, including neutron activation and many types of sample preparation systems coupled with detectors as diverse as mass spectrometers, ICP/MS, atomic absorption, and atomic fluorescence have been used to perform these analyses. As such, it can be very difficult to compare mercury analysis results obtained in different laboratories using different analytical techniques. The handling of samples can also significantly affect the measured results. A recent study found that the number of times a sample bottle had been opened could significantly affect the measured concentration.
Although analytical difficulties are responsible for some of the variability in crude oil mercury data, geological factors such as depositional environment and thermal history are a more important influence on the concentrations of mercury that are observed in currently produced oils. The mercury concentrations shown in FIG. 1 vary by more than a factor of 1000, which is indicative of the wide variety of environments from which these oils originate.
Mercury has come under increasing scrutiny in recent years because its presence in oil creates problems throughout the production, transportation, storage and refining systems. These problems include environmental concerns, contamination of refinery products, catalyst poisoning, corrosion of equipment, health risks for personnel, as well as expenses for disposal of contaminated tank sludge, cleaning of contaminated equipment, shipping restrictions, etc. These issues have led companies to search for methods of reducing the mercury content of crude oil.
Several processes for removing mercury from crude oil have been disclosed in U.S. Pat. Nos. 6,350,372, 6,537,443, 6,685,824, and 6,806,398 and in an article by Salvá (2010). All of these processes essentially involve reacting the crude oil with a sulfur compound to precipitate HgS and then separating the HgS particles from the oil by filtration or another technique for solids removal. In some cases, the process requires an additional filtration step prior to contacting the oil with the sulfur compound.
Such processes require the addition of reagent chemicals as well as filtration of the crude oil. The filtration step in particular is problematic because of filter plugging by other components in the crude oil such as waxes and sediments. The HgS will only comprise a small percentage of the total solids removed by filtration. The maintenance of a crude oil filtration unit can thus become cost prohibitive in terms of manpower, filtration media, and disposal.
The process disclosed in U.S. Pat. No. 8,080,156, by contrast, involves the use of natural gas to strip mercury from the crude oil. However, this process is only effective for the removal of elemental mercury. The efficiency of this process is thus limited by the ratio of elemental mercury to total mercury in the oil, and, in some cases, the efficiency of mercury removal can be very low.
In U.S. Pat. No. 9,574,140, incorporated herein in its entirety for all purposes, Applicant disclosed a method for determining the forms of mercury and their respective concentrations in a crude oil sample, which allowed for the development of a reaction rate expression for that particular crude oil feed and allowed the calculation of optimum design specifications for removal of mercury. Applicant found that crude oil can be heated to a temperature above 100° C., and held at that temperature for a specified period of time, to convert all of the forms of mercury in the oil into the elemental mercury form. The elemental mercury can then be stripped from the crude oil by e.g., flashing the hot oil and/or contacting it with a gas phase. Typically, temperatures of 180° C. or higher are required to achieve commercially useful reaction rates. The reaction rate is also strongly influenced by the composition of the crude oil. Condensates, for example, react more slowly than black oils. The ability to use the lower 100° C. temperature saves energy costs, and avoids undesired degradation of hydrocarbons.
Further improvements to methods of removing mercury are desired as companies are producing hydrocarbons from deeper and hotter reservoirs, which contain increasing concentrations of mercury, and in regions in the North Sea, Asia-Pacific, and South America, which also have elevated concentrations of mercury. Significant financial and environmental advantages exist for lowering the composition of the mercury in crude oil prior to sale.
Thus, what are needed in the art are better methods of removing mercury from crude hydrocarbons. Ideally, the methods will be simple, yet robust and applicable to all types of hydrocarbons, including crude oils, natural gas, and light condensates.